I'm not sure I see it. The horizontal structure is the low-contrast cloud formation.You have some noticeable uneven development in the sky area, how are you developing the film.
Now that you point it out, I can see it.I can easily spot it here on a ASUS semi tablet computer as well and it's definitely not natural, it's 5 or 6 maybe 7 darker vertical strips across the horizontal skyline. You'd possibly loose it with some simple dodging of the sky and background landscape.
My experience is that drizzle looks slightly different, not a series of spaced density irregularities You may be right about the internal of the tank causing streaming. Maybe others here using the same type of tank can comment (I know one has different issues).A possibility is that it's distant falling drizzle which can look like this, although usually not along the whole horizon.
I don't think there was any drizzle about that day, a bit misty, hence the red filter.A possibility is that it's distant falling drizzle which can look like this, although usually not along the whole horizon.
Are you agitating front-to-back or left-to-right? Fast or slow?
Perhaps changing to a couple of inversions every thirty seconds might help.
Do you use a water bath before the dev? Opinions are divided on the usefulness of a water bath in tanks.
Could it be that the slots in the SP-445 light-trap are behaving in the same way as 35mm sprocket holes sometimes do? This might be a possibility if the patches line up with the slots.
And yes, a bit of blackening is a good idea on the Intrepid.